Video keying may be performed with high speed video switches ("keyers") which select between two picture sources (video signals) during one frame period. By selectively switching between video signals, a composite video image may be produced. Generally, a keyer includes a switch and circuitry for generating a control (key) signal for the switch. The keyer selects between video signals based upon the status of the key signal.
Developments in the design of keyers have allowed relatively sophisticated combinations of two video signals. For instance, it is known in the art to utilize a key signal which represents a ratio of the first signal to the second signal. This type of key signal can be a function of various criteria. This signal allows the transition between the two video signals to be softened. A softened transition provides a more realistic composite image and reduces problems such as aliasing.
Video signals are generally comprised of two color difference signals and a luma signal. These signals are interconvertible with RGB representations. Various other types of video signals may also be used to provide a picture. In video signals, the symbol Y represents the luminance value and the symbols U and V represent two chrominance values. Generally, the U and V components represent the axes of a two dimensional space called the color plane. The practical range of the Y, U and V signals is preferably from -128 to +128 Which represents -3.5 volts to +3.5 volts in the analog domain with 8 bits per sample per component. The available values are 0-255. As per CCIR 601-2, "Encoding Parameters of Digital Television for Studios" parameter #8, the luminance signal will have 220 quantization levels with the black level corresponding to level 16 and peak white corresponding to level 235. Each color difference signal will have 225 quantization levels in the center part of the quantization scale with zero signal corresponding to level 128. Under this specification (CCIR 601-2), the Y channel analog signal will be 0V at 16 and +700 mV (milli-volts) at 235. Each color difference signal will be 0V at 128 and a negative -350 mV at 16 and a positive +350 mV at 240.
In order to provide a key signal that represents ratios of the first signal to the second signal (a signal that is more than bi-level (ON/OFF)), the combination of the two video signals must be by means of a multiplicative computation rather than a simple switching operation. This type of key signal is sometimes called a matte signal.
A circuit for providing a key signal or matte signal is discussed in "The Digital Chroma-Key" by V. G. Devreux (BBC Research Department, U.K.). In this reference, the chroma processor derives a key signal from the U and V signals. The circuit utilizes analog components for providing a multiplicative computation which generates the key signal.
"Digital Production Switchers" by Jacques Vallee (Thompson Video Equipment) discloses a digital chroma processor. The digital chroma processor includes a chroma key generator which utilizes digital multipliers and adders to develop a key signal. Similarly, U.S. Pat. No. 4,240,104 issued to Taylor et al. on Dec. 16, 1980 discloses a chroma key generator which is coupled to the U and V inputs. The generator receives the U and V signals from the first video signals and digitally performs various arithmetic manipulations to create a key signal. These generators are disadvantageous because the arithmetic computations by digital components are often inherently slow and because the processors can only be used with a single region of the color spectrum without adding additional hardware components.
In addition to keying, chroma processors also may perform a color correction operation. This operation allows a user to select certain colors in a video scene and then substitute new colors for the selected colors. In this application, U and V signals representative of the selected colors are chosen and the chroma processor replaces the U and V signals with U and V signals representative of the substitute colors. Also, the chroma processor may allow the user to select certain Y, U and V signals and replace the selected signals with different Y, U and V signals.
In U.S. Pat. No. 4,096,523, the color correction operation is performed by arithmetic processors which generate different Y, U and V values. Other prior art systems such as U.S. Pat. No. 4,727,412 utilize analog devices for providing color correction.
Analog prior art chroma processors are expensive and have limited accuracy and flexibility. Analog equipment is prone to drift and recalibration. Conventional digital prior art systems cannot process key signals and color correction data for different color regions without additional hardware. Accordingly, to have a competitive and acceptable system it is important to produce a system which is flexible.